Re: [PATCH] PERF(kernel): Cleanup power events V2

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Tuesday, October 26, 2010, Mathieu Desnoyers wrote:
> * Rafael J. Wysocki (rjw@sisk.pl) wrote:
> > On Tuesday, October 26, 2010, Mathieu Desnoyers wrote:
> > > * Peter Zijlstra (peterz@infradead.org) wrote:
> > > > On Tue, 2010-10-26 at 11:56 -0500, Pierre Tardy wrote:
> > > > > 
> > > > > +       trace_runtime_pm_usage(dev, atomic_read(&dev->power.usage_count)+1);
> > > > >         atomic_inc(&dev->power.usage_count); 
> > > > 
> > > > That's terribly racy..
> > > 
> > > Looking at the original code, it looks racy even without considering the
> > > tracepoint:
> > > 
> > > int __pm_runtime_get(struct device *dev, bool sync)
> > >  {
> > >         int retval;
> > > 
> > > +       trace_runtime_pm_usage(dev, atomic_read(&dev->power.usage_count)+1);
> > >         atomic_inc(&dev->power.usage_count);
> > >         retval = sync ? pm_runtime_resume(dev) : pm_request_resume(dev);
> > > 
> > > There is no implied memory barrier after "atomic_inc". So either all these
> > > inc/dec are protected with mutexes or spinlocks, in which case one might wonder
> > > why atomic operations are used at all, or it's a racy mess. (I vote for the
> > > second option)
> > 
> > No, it isn't.
> > 
> > > kref should certainly be used there.
> > 
> > No, it shouldn't.
> > 
> > Please try to understand the code you're commenting on first.
> 
> Please see my reply to Alan Stern:
> 
> http://www.spinics.net/lists/linux-omap/msg39382.html

I have and I'm still unimpressed. :-)

Please see my reply to that message.

Thanks,
Rafael